1. Field of the Invention
This invention relates generally to metamorphic buffers on small lattice constant substrates, for example AlInSb metamorphic buffer layers on Si, GaAs or InP substrates.
2. Description of the Related Art
Owing to the wide range of desirable electronic and optical properties of ternary, quaternary, and higher complexity semiconductor materials, there is an ongoing need to provide a suitable growth platform for semiconductor alloys for which no lattice matched substrate is readily available. Researchers and technologists have been quick to exploit the combinations of materials that have lattice constants close to those of readily available substrate materials. However, many interesting and potentially high performance device structures, including emitters, detectors, and high speed electronic devices, face fabrication challenges due to the lack of a suitable substrate on which to grow them.
For example, for III-V compound semiconductors, the substrates available in reasonable quantity with high metallurgical quality at affordable cost are limited to a few of the binary materials, including GaAs, InP, InAs, InSb, GaSb, and GaP. To date, no high quality substrates of any of the ternary III-V alloys are commercially available although many of these alloys are of interest for device structures. However, semiconductor devices in which the active region of the device is grown directly onto a lattice-mismatched substrate usually contain a large density of metallurgical (structural) defects, leading to poor device performance. Thus, if full advantage is to be taken of the wide range of electronic and optical properties of III-V alloys, a suitable growth platform is highly desirable.
Various attempts have been made to address this problem. For example, thin strained films (i.e., strained materials that have a different lattice constant than the substrate) can be grown up to thicknesses usually not exceeding 10-50 nm. However, devices often need thicker layers on the order of 1-10 micron. For these thicknesses, typical strains of 0.5 to 2% are extremely difficult, if not impossible, to accommodate in the crystal. The strain relaxes and dislocations and defects form that are generally deleterious to the device operation.
Another approach to achieving flexibility in the lattice constant of the “substrate” material is the use of appropriate metamorphic (strain-relaxed) buffer layers in various forms. These metamorphic layers are grown on a commercially available substrate in such a way that they relax to a lattice constant suitable for the epitaxial growth of the desired device structure. Ideally, one can in effect achieve a ternary (or quaternary or more complex) alloy substrate, not by bulk growth techniques, but by epitaxial growth processes. However, previous demonstrations of metamorphic buffer layers are typically on substrates that have a lattice mismatch of not more than 4% with respect to the desired device structure. For example, AlInSb metamorphic buffer layers have been demonstrated on GaSb substrates. However, the buffer layer contacting the GaSb substrate typically is an AlSb layer. The GaSb substrate has a lattice constant of 6.096 angstroms; the AlSb layer has a lattice constant of 6.136 angstroms. This represents a lattice mismatch of only 0.66% between the GaSb substrate and the AlSb layer.
It is generally accepted that it is difficult to change the lattice constant by significantly more than this within a short distance (e.g., typically 1-10 micron of material thickness) if an acceptable dislocation density is desired (e.g., typically below 100 million/cm). Unfortunately, many of the common substrates, including Si, GaAs and InP, will require a change in lattice constant of this magnitude. For example, for an active region that has a lattice constant of 6.1 angstroms, InP, GaAs and Si have lattice mismatches of approximately 3.9%, 7.9% and 12.3%, respectively. In contrast, the lattice constant of GaSb is approximately 6.1 angstroms, yielding only minimal mismatch if any. There is a marked lack of demonstrations of mismatches of more than 4%.
Thus, there is a need for a growth platform that is matched to the larger lattice constant of an active device but based on a smaller lattice constant substrate.